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Abstract

Background

Stavudine continues to be used in antiretroviral treatment (ART) regimens in
many resource-limited settings. The use of zidovudine instead of stavudine
in higher-risk patients to reduce the likelihood of lactic acidosis and
hyperlactatemia (LAHL) has not been examined.

Conclusions

Stavudine exposure, female sex, and higher BMI are strong, independent
predictors for developing LAHL. Patients with risk factors for lactic
acidosis have less LAHL while on zidovudine- rather than
stavudine-containing ART. Switching patients from stavudine to zidovudine is
protective. Countries continuing to use stavudine should avoid this drug in
women and patients with higher BMI.

Introduction

Lactic acidosis is a potentially fatal side effect of nucleoside analog reverse
transcriptase inhibitors (NRTIs) [1], [2], which are commonly used in combination antiretroviral
therapy (ART). This complication is related to NRTI-induced mitochondrial toxicity
possibly due to structural similarities between mitochondrial DNA polymerase and
HIV-reverse transcriptase (the target of NRTIs) [3]. The incidence of lactic
acidosis among patients on ART ranges from 1–4 per 100 patient years in
resource-rich settings and is as high as 10 per 100 patient years in sub-Saharan
African cohorts [4], [5], [6], [7], [8], [9], [10], [11]. The lactic acidosis case-fatality rate in
resource-limited settings can be as high as 60% [12].

Of the NRTIs, the dideoxynucleosides (stavudine and didanosine) confer the highest
risk of lactic acidosis [1], [2], [5]. While stavudine is rarely used in resource-rich settings
and is no longer recommended by the World Health Organization for initial treatment
of HIV-1 infection [13], it remains an important component of standard ART
regimens in many resource-limited countries, largely due to cost [14], [15]. In South Africa
where stavudine is no longer recommended for use in first-line therapy, patients
receiving stavudine-containing ART are only switched if there is evidence of
toxicity, again because of financial constraints. In settings where stavudine is
widely prescribed, lactic acidosis is a frequent cause of morbidity and mortality
[1], [2], [4], [5], [6], [7], [8], [9], [10], [16] and is
associated with high losses to follow-up and treatment discontinuation [15].

Until April 2010, first-line therapy in South Africa included stavudine, lamivudine,
and either efavirenz or nevirapine. Based on observational findings from a
site-specific study that identified a high incidence of lactic acidosis in women
with BMI≥28 kg/m2, in August 2005 the HIV Clinic at McCord Hospital in
Durban, South Africa substituted zidovudine for stavudine in initial ART for
patients with these two risk factors [7]. The policy continued until
March 2007, when the clinic was accredited as a Department of Health site and
required to follow Department of Health guidelines for ART, including the use of
stavudine as part of initial regimens.

To evaluate the impact of risk factor-guided selection of initial therapy, we
compared the combined incidence of lactic acidosis and hyperlactatemia among
treatment-naive patients initiating stavudine-containing therapy with those starting
zidovudine-containing therapy. We hypothesized that risk-factor-guided ART
(initiating women with BMI≥28 kg/m2 on zidovudine rather than stavudine) would be
associated with decreased incidence of lactic acidosis and hyperlactatemia. We also
assessed predictors of lactic acidosis and hyperlactatemia.

Methods

Ethics statement

Ethics approvals were obtained from the McCord Hospital Medical Ethics Research
Committee and from the Partners Healthcare Institutional Review Board (Boston,
MA). Given the nature of the study (retrospective chart review), the requirement
for informed consent was waived by the ethics committees.

Study design and population

Patient data were collected from the outpatient HIV clinic at McCord Hospital in
Durban, South Africa which has initiated over 8000 patients on ART. During the
study period, initial ART included two NRTIs and one NNRTI: stavudine (30 mg
twice daily; 40 mg twice daily if weight >60 kg) or zidovudine plus
lamivudine and either efavirenz or nevirapine.

The study population included antiretroviral (ARV)-naïve, HIV-infected
adults (age ≥18 years) with baseline laboratory data and at least one
follow-up visit after ART initiation. Two retrospective cohorts were identified.
The first cohort included patients who initiated stavudine-containing therapy
between July 2004 and March 2007. The second cohort included patients who
initiated zidovudine-containing ART between July 2004 and March 2007. Both
cohorts included patients who initiated ART between August 2005 and March 2007
when the clinic made women with BMI≥28 kg/m2 eligible for initiation of
zidovudine -containing therapy or for regimen switch from stavudine to
zidovudine.

Outcomes and their measurement

The primary outcome was event-free survival defined as the time from treatment
initiation to development of lactic acidosis (symptomatic or asymptomatic) or
hyperlactatemia (symptomatic or asymptomatic) (Table 1). Lactic acidosis and hyperlactatemia
were defined based on AIDS Clinical Trials Group criteria [20]. Lactic acidosis is defined
as having a lactate level above the upper limit of normal (4.4 mmol/L) along
with evidence of acidosis (bicarbonate level <20 mmol/L or pH<7.35).
Hyperlactatemia is defined as a lactate level greater than the upper limit of
normal without evidence of acidosis. Cases of symptomatic lactic acidosis or
hyperlactatemia met the above criteria and had new, otherwise unexplained
symptoms of nausea, vomiting; abdominal pain, discomfort, or distention;
increased hepatic transaminases; fatigue; dyspnea; weight loss (≥5%);
or muscle weakness. Because these patients were ambulatory and often did not
have repeat measurements, confirmed elevation of lactate levels was not required
if at least two symptoms were present.

Blood was drawn for lactate levels without use of a tourniquet and specimens were
transported on ice and processed within four hours (Beckman Coulter, Synchron
systems, California, USA). A handheld lactate detection device, a reliable proxy
for serum samples, was introduced in 2006 (Accutrend model #3012522) [21], [22] and was
used for initial screening in addition to serum lactate testing.

Outcomes were classified from a review of the medical records of patients
initiating ART during the study period. This review was facilitated by the
requirement that clinicians record the reason for any change or discontinuation
in ARV regimen from an electronic pull down menu. For patients who had a regimen
change noted in the electronic record, paper charts were reviewed for the
following: 1) documentation of a regimen change due to lactic acidosis; 2)
documentation of a regimen change and signs or symptoms that could be consistent
with lactic acidosis or hyperlactatemia (nausea, vomiting, abdominal discomfort,
bloating, increased hepatic transaminases, fatigue, dyspnea, weight loss, muscle
weakness); 3) documentation of a regimen change without specific reason listed;
4) death. Serum lactate test results for all study patients were reviewed. A
lactate value above 3 mmol/L prompted review of the medical record for symptoms
of hyperlactatemia, all available lactate values, other possible causes for
symptoms or elevated lactate levels, and clinical outcome. Data were abstracted
using standardized abstraction forms (LM, AE, JH). For a subset of patients
(n=20), two physicians carried out the abstractions with
100% agreement on outcome classification (JH, LM). Cases with unclear
outcomes were adjudicated by a senior clinician (RG).

We also identified patients for whom clinicians had changed ART due to peripheral
neuropathy, lipodystrophy, high BMI, and drug resistance, as indicated in the
electronic medical record. We identified patients with regimen switch for
clinical suspicion of LA or HL but who did not meet criteria. These subjects
were not censored at change in regimen but followed out to a total two years of
follow-up from treatment initiation. In addition, we identified subjects who
changed clinic site, stopped ART, died or were lost to follow up.

Covariates

Covariates were obtained from paper chart abstractions and included weight at
treatment initiation (within 3 months) and height. Weights obtained during
pregnancy were excluded. BMI was calculated (kg/m2) for all subjects
in whom height and baseline weight were available. Sex, date of birth and
baseline (the last value prior to ART initiation, or within 2 weeks) CD4 count,
creatinine, hemoglobin, alanine aminotransferase, and albumin were extracted
from the electronic record or the paper chart. All specimens were processed
using standardized methods at laboratories in Durban.

Time to event or censor

The primary outcome was 2-year event-free survival (EFS) defined as the time from
treatment initiation up to development of lactic acidosis or hyperlactatemia.
Patients were also censored for loss to follow-up, change in clinic site,
termination of treatment, death, or at study end. All others were followed for
two years or until the primary outcome. Time on stavudine and zidovudine was
calculated from start and stop dates entered by clinicians in the medical
record.

Analysis

We calculated crude incidence rates for the combined primary outcome (LAHL), the
combined incidence of peripheral neuropathy and lipodystrophy, death, and
loss-to-follow-up. Confidence intervals for event rates based on initial therapy
with stavudine or zidovudine were estimated using methods for exact binomial
confidence intervals and compared using Chi-square tests [23]. Kaplan-Meier curves were
plotted for event-free survival based on initial treatment and rates were
compared using the log-rank test statistic. Univariate and multivariate analyses
using Cox proportional hazards (PH) regression models were utilized to assess
the effect of treatment on time to event [24]. We evaluated time on
zidovudine or stavudine as a time-varying covariate to account for variable time
on drug among patients whose regimens were switched in the absence of the
outcome of interest (e.g. switch for peripheral neuropathy or increased BMI).
Covariates for multivariate analysis were selected based on significance (p
value<0.05) in univariate analysis and significant covariates in the
literature. CD4 count was modeled as a continuous variable with the effect size
reported per 10-cell increment. BMI was modeled on a natural logarithmic scale
with effect size reported per 30% shift. In the full model, BMI deviated
from the proportional hazards assumption and was modeled with a time-dependent
association for early (within the first year) and late (after one year) failure.
In subgroup analysis, BMI followed proportional hazards. All statistical
analyses were carried out using SAS version 9.2 for Windows.

Results

Baseline patient and disease characteristics

Two-thousand-sixty-two patients contributing 2747 person years of follow-up were
included in the study. The median age was 34.7 years (IQR 29.8, 40.6) and
60% were women. Eighty-nine percent initiated therapy with a
stavudine-containing regimen. One-hundred sixty one (77%) of those who
were initiated on a zidovudine-containing regimen were started because of higher
BMI or other perceived lactic acidosis risk factors. The remaining patients were
initiated on zidovudine because of pre-existing lipodystrophy (<1%),
peripheral neuropathy (<1%), pregnancy (10%), or unknown reason
(10%).

Median CD4 count at entry was 80 cells/mm3 (IQR 29–142). Median BMI for
subjects with complete data (88% had documented weight at entry,
76% had documented height) was 22 kg/m2 (IQR 20, 26). Compared with those
initiated on a stavudine-containing regimen, patients started on zidovudine were
older, more likely to be female, had a higher BMI, higher CD4 cell count, higher
albumin and higher hemoglobin. Other characteristics are described in Table 2.

Outcomes for full cohort

In intention to treat analysis, combined incidence of LAHL was 3.2/100 PY in the
stavudine- and 0.34/100 PY in the zidovudine-initiated group (RR 9.26,
95% CI 1.28–66.93, p=.007). There were 36
lactic acidosis and 43 hyperlactatemia events in the stavudine group. In
contrast, there was 1 lactic acidosis event in the zidovudine group: this
occurred in a woman who initiated zidovudine-based therapy because of high BMI
(31 kg/m2); one year later, she was switched to stavudine because of anemia;
after eight months on stavudine-containing ART, she was diagnosed with lactic
acidosis. Mortality due to causes other than LAHL was 8.3% and
2.8% in stavudine- and zidovudine-initiated patients, respectively
(RR=2.89, 95% CI:1.45–5.78,
p=0.001). The combined incidence of physician-reported
peripheral neuropathy and lipodystrophy was 16.8/100 PY in stavudine- and
0.34/100 PY in zidovudine-initiated groups (RR=59.84,
95% CI: 8.36–428.12, p<0.001). Loss to follow-up was equivalent
between the two groups (RR=1.42, 95% CI:
0.68–2.96, p=0.35). (Table 3)

Incidence of mitochondrial toxicity, death and loss to follow-up by
initial treatment.

In univariate Cox proportional hazards analysis, stavudine in the initial
treatment regimen, female sex, higher BMI, and higher baseline albumin were each
associated with increased risk of LAHL (Table 4). The Kaplan Meier curve for time to
LAHL based on initial treatment regimen is shown in Figure 1 (p=.006).

Univariate and multivariate Cox Regression Analysis for time to
lactic acidosis or hyperlactatemia.

In multivariable Cox PH regression to assess predictors of event-free survival,
hemoglobin and ALT were removed but age and CD4 cell count were included because
prior data and a priori knowledge suggested an association with
lactic acidosis [1]. Creatinine was into the full model when it was found
to be significant in subgroup analysis. The adjusted hazards of experiencing
LAHL was higher for those on stavudine (HR=14.31,
95% CI 5.79–35.30), women (HR=3.41,
95% CI: 1.89–6.19), subjects with higher BMI in the first year
(HR=3.21, 95% CI: 2.16–4.77), higher albumin
(HR=1.04, 95% CI:1.01–1.07), higher
creatinine (HR=1.63, 95% CI 1.12–2.36), or
lower baseline CD4 cell count (HR=0.96, 95% CI:
0.92–1.00) at baseline (Table 5). Among those initiated on stavudine, the hazards of
experiencing LAHL was lower for those who were switched to zidovudine during
follow-up (HR 0.15, 95% CI 0.06–0.35).

Multivariate Cox Regression Analysis for time to lactic acidosis or
hyperlactatemia limited to women with BMI≥25 kg/m2.

Of the 194 women with higher BMI who initiated stavudine-inclusive therapy, 137
were switched to zidovudine for reasons other than LAHL. Baseline
characteristics (age, BMI, CD4 cell count, creatinine albumin, ALT, hemoglobin)
were not significantly different from women with higher BMI initiated on
stavudine-treatment who did not switch treatment arms. Women were switched for
high BMI (79, 56%); lipodystrophy, peripheral neuropathy, or these plus
elevated BMI (47, 34%); lab values and/or symptoms suggestive of
hyperlactatemia that did not meet criteria for LAHL (7, 5%); and the
remainder were switched for anemia, pregnancy, rash or other reasons. These
participants subsequently contributed an additional 131.7 woman-years of
follow-up (mean 1.1 years ±0.5) during which there were 5 LAHL events
(3.8/100 woman years). All but one event occurred within 2–8 weeks of
switching off stavudine after an average of 0.6±0.4 years on stavudine,
suggesting that the recent and cumulative stavudine exposure contributed to the
toxicity. When controlling for other LAHL risk factors, switch to zidovudine
conferred 80% lower hazards of LAHL for this subgroup (HR 0.21,
95% CI 0.07–0.64, p=.006). The remainder of
women in this subgroup of obese women, initiated on stavudine and switched to
zidovudine, included two who subsequently had anemia and two who died; the rest
were followed until the end of the study, change in service provider, or a
maximum of two years of follow-up without adverse events.

Discussion

In our study of 2062 HIV-positive patients who initiated ART, stavudine use confers a
fourteen-fold increased risk of developing hyperlactatemia or lactic acidosis when
controlling for other risk factors (HR 14.31, 95% CI 5.79–35.30). Other
risk factors for the primary outcome of LAHL were female sex, higher baseline BMI,
higher baseline creatinine or albumin, and lower initial CD4 cell count. For
patients who started a stavudine-containing regimen, switching to zidovudine was
associated with 85% lower hazards of developing LAHL (HR 0.15, 95% CI
0.06–0.35). For the high-risk subgroup of women with BMI≥25 kg/m2 who
initiated therapy on stavudine-containing ART, switch to zidovudine was also
protective when controlling for other risk factors (HR 0.21, 95% CI
0.07–0.64).

Our study adds to the literature by demonstrating that female sex is a strong
independent risk factor for developing LAHL [1], [4], [7], [11], [16], [17], [18]. Higher weight has been
associated with these outcomes in prior studies, but this is the first to confirm a
relationship with BMI and LAHL when controlling for other covariates [1], [11], [16], [17], [18]. For every
30% change in BMI (i.e. 18 to 23 kg/m2 or 24 to 31
kg/m2), we observed a three-fold increase in the LAHL rate
(HR=3.21, 95% CI: 2.16–4.77). For the full
dataset, the effect was only significant in the first year of follow-up which may
reflect increased risk earlier in treatment or insufficient power to detect an
association after the first year. During the study, patients with weight >60 kg
received 80 mg of stavudine daily, which has been linked to worse mitochondrial
toxicities compared to use of 60 mg [25]. We were unable to control for stavudine dose; thus, the
high incidence of LAHL in patients with higher BMI might be related to higher
stavudine dose. This possibility is supported by observations that patients on
higher dose stavudine (40 mg twice daily) have a higher incidence of elevated
lactate than those who receive lower doses (20 or 30 mg twice daily) [16], [25]. However, given
that multiple studies involving varying stavudine dose have found an association
between higher weight or BMI and lactic acidosis [7], [11], [17] while, in some cases,
controlling for dose [16], [18], it is unlikely that drug dosing explains the entire
effect. Furthermore, a three-fold increase in hazards of LAHL was observed in our
subgroup analysis of women with BMI≥25 kg/m2 (who likely received uniform
stavudine dosing).

Higher creatinine was associated with increased hazards of LAHL, about 25% per
1 mg/dL unit increase in creatinine. This risk factor has not been reported in prior
univariate analyses and has not been included in studies that control for other risk
factors, but is not unexpected given the kidney's role in lactate metabolism
[26]. We also
found that higher albumin is associated with an increased risk of LAHL and a small
protective effect of higher CD4 cell count at treatment initiation (4%
decrease in hazards for each 10-point increase in baseline CD4 cell count). Two
other studies have also found an association between CD4 cell count and lactic
acidosis [1],
[27]. Each of
these associations (CD4 cell count, creatinine and albumin) was small with
confidence intervals close to one.

The Lactic Acidosis International Study group showed an association of older age (age
>40 years) with the development of LAHL. This was not seen in our cohort nor in
other studies based in Southern Africa [16], [17], [18]. The majority of their subjects
were from Europe and the Americas with an older age distribution than in our study
(mean of 42 years for cases vs. 35 years).

For the subgroup of women with higher BMI, stavudine use, when controlling for other
risk factors, remained associated with a significant increase in risk of LAHL.
Switching these women to zidovudine conferred an 80% reduction in hazards of
LAHL. These data suggest that higher-risk individuals should be switched off
stavudine-based therapy in order to reduce adverse events.

There are several limitations to this study. The two treatment groups were quite
different as demonstrated in Table
1. In our model we were able to control for the variables in the model,
but not for unmeasured confounders (e.g. HIV clinical stage). Clinicians may have
been more likely to test for hyperlactatemia in patients on stavudine resulting in a
detection bias. However, 14% of subjects initiated on stavudine and
18% of subjects initiated on zidovudine had at least one serum lactate level
checked during the study period, suggesting that serum lactate testing was not
biased towards subjects on stavudine. We do not think deployment of the handheld
lactate machine for screening (introduced in 2006) differentially affected case
finding between the two groups. Any patient with a positive handheld device test
required confirmatory serological testing; as above, testing rates were not higher
for subjects initiated on stavudine. Our data, in combination with prior data
evaluating risk factors for lactic acidosis and hyperlactatemia, strongly suggest
that women and patients with higher BMI treated with stavudine are at high risk for
developing LAHL. In addition, our data demonstrate that using zidovudine rather than
stavudine, even among patients at highest risk for mitochondrial toxicities,
dramatically reduces the risk of developing lactic acidosis. Further, for patients
initiated on stavudine-based therapy, switching to zidovudine is protective.
Although a recent Cochrane Review concluded there is no difference in treatment
outcomes (toxicity, death, disease progression) for stavudine- compared with
zidovudine-based ART, the randomized-controlled trials on which their analyses were
based included patients from North America, the Caribbean, Australia and China [28]. However,
most studies observing high rates of mitochondrial toxicities include patients from
sub-Saharan Africa.

As of April 2010, first-line ART in South Africa includes tenofovir, lamivudine, and
either efavirenz or nevirapine [29]. However, because of drug shortages many clinics remain
unable to initiate all patients on tenofovir-based therapy and are not able to
routinely switch patients to tenofovir-containing regimens. Thus, in South Africa
(and other countries still using stavudine) these findings will be helpful in
identifying patients who are at highest risk for stavudine-induced complications.
While all patients will benefit from using alternatives to stavudine, avoiding this
drug in women and patients with higher BMI may offer an effective and practical
strategy for reducing the incidence of lactic acidosis and hyperlactatemia until
countries can completely eliminate use of this agent.

Acknowledgments

We would like to acknowledge the patients, clinicians and monitoring and evaluation
staff at Sinikithemba. We would also like to acknowledge data capturers and McCord
volunteers Lisa Bevilacqu, Anthony Sawyer, Winn Seay, Mary Gallo, Dr. Hannah
Willoughby, and Dr. Eileen Scully. We are also grateful to Dr. Heather Ribaudo of
the Harvard University Center for AIDS Research Biostatistical Core and Dr. Roger
Davis for statistical support.

Footnotes

Competing Interests: The authors have declared that no competing interests exist.

Funding: Dr. Matthews' work was supported by the Mark and Lisa Schwartz Family
Foundation and by a postdoctoral fellowship in tropical infectious diseases from
the Burroughs Wellcome Fund/American Society for Tropical Medicine and Hygiene.
Dr. Gandhi is supported by NIH R01 AI066992-04A1 and NIH G08LM008830-01 and by
grants to the AIDS Clinical Trials Group (NIH U01 AI 694722) and the Harvard
University Center for AIDS Research (NIH 2P30 AI060354-06). Dr. Bangsberg was
supported by NIH grant MH K-24 87227. The authors are also grateful to Dr.
Heather Ribaudo of the Harvard University Center for AIDS Research
Biostatistical Core (NIH #AI060354). The funders had no role in study design,
data collection and analysis, decision to publish, or preparation of the
manuscript.